1. Field of the Invention
The present invention relates to a coated carrier and a method of manufacturing the coated carrier.
2. Description of the Related Art
There has been used a two-component developer composed of a small particle size toner having a volume average particle size of 5 to 7 μm and a small particle size carrier having a volume average particle size of 30 to 40 μm in an image forming apparatus using electrophotography. In order to make durability and environmental stability of the two-component developer excellent, it is important to charge a developer stably in the image forming apparatus. Therefore, for development of carriers, searching of core materials and coating resin materials constituting a carrier and optimization of coating amount have been performed.
A color developer described in Japanese Unexamined Patent Publication JP-A 4-177369 (1992) is a developer containing a no, magnetic color toner and a carrier and contains a carrier whose carrier core is coated with an electrically insulating resin of 0.1 to 5.0% by weight.
Additionally, a magnetic carrier described in Japanese Unexamined Patent Publication JP-A 8-44118 (1996) is composed of a magnetic core particle and a resin coating layer, and the resin coating layer is mainly formed of a thermoset resin. In Japanese Unexamined Patent Publication JP-A 8-44118, a thermoset resin such as a silicone resin, a thermoset acrylic resin, a phenolic resin, an urethane resin, or a thermoset polyester resin is used, and thereby making adhesiveness and wear resistance of a resin coating layer excellent. As a result, a carrier has a durable structure as a whole.
However, even in the case of a carrier using a thermoset resin for a coating layer, there still remains a problem that the coating layer comes off and carrier characteristics change due to a long period of use. In particular, in a small particle size toner, an external additive having relatively large particle size of 100 to 300 nm is often used. In such a case, the coating layer on a carrier surface is polished by the external additive, and therefore carrier characteristics in an initial state can not be maintained, thus lowering durability of the carrier.
Such a method is possible to handle the above mentioned problem that the coating layer is thickened and thereby the carrier characteristics are not influenced even in a case where the coating layer is worn away, however, in the method, there is a problem such that it becomes difficult to control so that thickness is uniform along with thickening the coating layer, for example, when a core material is coated with the thermoset resin, several pieces of carriers aggregate, the aggregated carriers are disintegrated, thus generating a fracture surface on the coating layer.